Immobilization of Ni(ii) complex on the surface of mesoporous modified-KIT-6 as a new, reusable and highly efficient nanocatalyst for the synthesis of tetrazole and pyranopyrazole derivatives

In this paper, KIT-6@SMTU@Ni was successfully synthesized via a new method of Ni(ii) complex stabilization on modified mesoporous KIT-6, as a novel and green heterogeneous catalyst. The obtained catalyst (KIT-6@SMTU@Ni) was characterized using Fourier transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) calculation, X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), energy-dispersive X-ray spectroscopy (EDS), X-ray mapping, thermogravimetric analysis (TGA) techniques and scanning electron microscopy (SEM). After complete characterization of the catalyst, it was successfully used for the synthesis of 5-substituted 1H-tetrazoles and pyranopyrazoles. Moreover, tetrazoles were synthesized from benzonitrile derivatives and sodium azide (NaN3). All tetrazole products were synthesized with high TON, TOF and excellent yields (88–98%) in a reasonable time (0.13–8 h), demonstrating the efficiency and practicality of the KIT-6@SMTU@Ni catalyst. Furthermore, pyranopyrazoles were prepared through the condensation reaction of benzaldehyde derivatives with malononitrile, hydrazine hydrate and ethyl acetoacetate with high TON, TOF and excellent yields (87–98%) at appropriate times (2–10.5 h). KIT-6@SMTU@Ni could be reused for five runs without any re-activation. Significantly, this plotted protocol has prominent benefits, such as applying green solvents, the use of commercially available and low-cost materials, excellent separation and reusability of the catalyst, short reaction time, high yield of products and a facile work-up.


Introduction
Green chemistry has appeared over the past few decades and has enabled chemists to comprehend these concepts and use them to design advanced syntheses. Chemists have realized the devastating effect of the chemical industry on the environment and human health so that, accordingly, they are involved in trying to minimize it. 1,2 In this sense, catalysis is one of the principal factors in "green chemistry" and the development of safe environmental catalysts is one of the most important challenges for chemists. 3,4 Therefore, a stable and "green" catalyst should have clear characteristics, such as high selectivity and activity, low preparation cost, high stability, effective recovery, and reusability. In this regard, heterogeneous catalysts have received outstanding consideration thanks to their extraordinary ability to increase the rates of organic reactions. 1 In recent years, the stabilization of homogeneous catalysts on solid supports (various nanoparticles) has been extended to the design of heterogeneous catalysts. 2,5 In this regard, it is worth mentioning that decreasing the particle size would result in increasing its surface area, which would lead to a high capacity for catalyst loading. 6 Fortunately, to date, various supports, including biochar nanoparticles, zeolites, mesoporous silica materials, iron oxide, carbon nanotubes, metal-organic frameworks, graphene oxide, boehmite nanoparticles, ionic liquids and microporous organic polymers have received a lot of attention for synthesizing heterogeneous catalysts. [7][8][9][10][11][12][13][14][15][16][17][18][19] According to the IUPAC denition, mesoporous materials have pore sizes between 2 and 50 nm. These materials have particular characteristics, such as high surface area, excellent surface performance, orderly porosity, high pore volume, and good mechanical and chemical stability. Furthermore, one known member is mesoporous silica. Mesoporous silica materials are a family of materials that were rst discovered in 1992. Among them may be mentioned MCM-48, MCM-41, KIT-1, KIT-6, SBA-15, SBA-16, and MCF-7. [20][21][22][23][24][25][26][27] Among diverse catalyst supports, KIT-6 has superior benets, such as an extremely uniform pore distribution, adjustable pore size, dense silanol heterocyclic organic compounds, which include various potentials such as intrinsic convergence, reduction in time, atom economy, savings in cost and energy, environmental benets, convergence and operational simplicity. For example, pyranopyrazole derivatives can be synthesized from the fourcomponent condensation of hydrazine hydrate, ethyl acetoacetate, aldehyde derivatives, and malononitrile. [48][49][50][51] Signicantly, pyranopyrazoles have been used as potential inhibitors of human CHK-1 kinase, and for their insecticidal, pharmaceutical, antimicrobial, anticancer, anti-inammatory, antiviral, analgesic, and vasodilator activities. 41,[52][53][54][55][56][57][58] In this sense, the aim of the present article is to design an effective and convenient method for the stabilization of a new complex of nickel with s-methyl isothiouronium sulfate on KIT-6 (KIT-6@SMTU@Ni) in the synthesis of tetrazoles and pyranopyrazoles.

Materials and instruments
All starting materials and solvents employed in this project were purchased from Iranian companies, Merck and Aldrich. The non-ionic surfactant Pluronic P123 triblock copolymer, 3chloropropyltrimethoxy silane (CPTMS), tetraethylorthosilicate (TEOS) and other chemicals used in the study were purchased from Aldrich and Merck. The catalyst was analyzed using IR spectra of the samples prepared with a KBr disk using a Bruker VERTEX 70 model FT-IR spectrophotometer. X-ray diffraction (XRD) patterns were prepared with a Co radiation source (l = 1.78897 Å) operated at 40 keV. The thermogravimetric analysis (TGA) data were obtained with a Shimadzu DTG-60 analyzer. The morphology was investigated by measuring SEM using a TESCAN MIRA FESEM microscope. The Brunauer-Emmett-Teller (BET) surface area (S BET ) was calculated from the linearity of the BET equation.

KIT-6 synthesis
KIT-6 mesoporous silica was prepared according to published articles. 33,59 Briey, Pluronic P123 copolymer (4 g) was added to HCl solution (150 mL, 0.5 mol L −1 ) and then stirred for at least 3 h at 35°C until complete dissolution. Aerward, n-butyl alcohol (4.95 mL) and TEOS (9.2 mL) were injected into the above solution, followed by stirring for 24 h at 35°C. Subsequently, the mixture was moved into a Teon-lined autoclave and then heated for 24 h at 100°C. Finally, the white solid product was ltered without washing, and dried at 100°C for 12 h. The material was calcined at 550°C for 4 h.

Modication of KIT-6 with 3chloropropyltrimethoxysilane (CPTMS)
In this step, a mixture of 1.5 mL of 3-chloropropyltrimethoxysilane and 1.0 g of KIT-6 was reuxed in 40 mL of toluene at 100°C for 24 h. Aerward, the material was ltered and washed with ethanol and n-hexane several times and, nally, dried in an oven at 50°C to obtain the modied KIT-6 (KIT-6@CPTMS).

Preparation of nickel catalyst (KIT-6@SMTU@Ni)
In the last step, 1.0 g of KIT-6@SMTU was dispersed into 40 mL of ethanol and then 2 mmol of Ni(NO 3 ) 2 $6H 2 O was added and, nally, the mixture was reuxed for 24 h at 80°C. The obtained catalyst was ltered, washed with ethanol and deionized water, and then dried for 12 h at 50°C to obtain KIT-6@SMTU@Ni (Scheme 1).

Results and discussion
Herein, the preparation and characterization of SMTU@Ni on KIT-6 are reported for the rst time. Its application was studied for the synthesis of 5-substituted 1H-tetrazoles and pyranopyrazoles as novel heterogeneous and reusable catalysts.

Low-angle XRD pattern studies
In order to assess the order of the mesoporous structure of KIT-6, and the material and characterization of the KIT-6@SMTU@Ni catalyst, the samples were characterized using the X-ray diffraction (XRD) method. The low-angle XRD patterns for KIT-6 as a support and the KIT-6@SMTU@Ni catalyst are shown in Fig. 1. It turns out that two peaks of (211) and (220) are recorded for KIT-6 ( Fig. 1a), which correspond to the XRD pattern of KIT-6 containing regular cavities with Ia3d cubic symmetry. 33,59 The XRD pattern for the catalyst (Fig. 1b) shows that the mesoporous structure of KIT-6 remains well preserved aer modication; but the peak intensities are reduced. The decrease in the intensity of the XRD peaks is due to a change in the dispersion pattern and in the pore wall aer the functionalization process.

SEM photographs
The SEM technique provides information about a sample, including the topography of the sample, surface properties, shape, size, and placement of particles on the body surface and the composition of the components that make up the sample. In this research, the morphology of the samples was checked by applying the SEM technique. SEM images of KIT-6 and the KIT-6@SMTU@Ni catalyst are shown in Fig. 2. As can be seen, there is no remarkable change in the morphology of the catalyst surface compared to the morphology of KIT-6. This observation conrms that the nickel complex is stabilized in the cavities of KIT-6 and its morphology has not changed. The size of three particles from the KIT-6 support was calculated randomly, and their diameters were in the range of 44.30-68.53 nm. Also, the sizes of two particles from the KIT-6@SMTU@Ni catalyst were calculated randomly, and their diameters were in the range of 26.28-40.01 nm.

Energy dispersive X-ray analysis and elemental mapping
EDS analysis was undertaken to show the presence of elements in the structure of the KIT-6@SMTU@Ni catalyst (Fig. 3). As depicted, the EDS result of this catalyst (KIT-6@SMTU@Ni) shows the presence of silicon, oxygen, carbon, nitrogen, and also nickel species. Moreover, the elemental X-ray mapping of the catalyst (KIT-6@SMTU@Ni) conrmed that the elements (oxygen, carbon, silicon, nitrogen, and Ni) are distributed homogeneously on the catalyst surface (Fig. 4). These results indicate that the nickel complex has been successfully immobilized on the KIT-6 support.
Moreover, the exact amount of Ni which was loaded on KIT-6 was calculated using AAS analysis (0.23 × 10 −3 mol g −1 ).

Thermogravimetric analysis studies
Graphs from TGA analysis show the change in mass of the sample based on a function of temperature where different molecules are adsorbed by heat at different temperatures. As illustrated in the TGA diagram of the KIT-6@SMTU@Ni catalyst (Fig. 5), the weight decrease from 35°C to 200°C can be assigned to the removal of adsorbed organic solvents and water in the mesoporous materials. 60,61 The weight loss observed in the temperature range of 200°C to 700°C can be attributed to the disintegration of the immobilized organic compounds. These results indicate that the organic groups have been successfully stabilized on the KIT-6 surface.

FT-IR spectra
The type of functional groups present in KIT-6 and the KIT-6@SMTU@Ni catalyst can be illustrated by FT-IR spectroscopy. In the FT-IR spectrum for KIT-6 nanostructures (Fig. 6), a broad band in the 3445 cm −1 region is relevant to the stretching vibrations and the absorption spectrum at about 1637 cm −1 is related to the exural vibrations of the surface OH groups. 62 The absorption band is at about 1079 cm −1 for the asymmetric stretching vibrations of the Si-O-Si groups. The absorption spectra of the symmetric stretching vibrations of the Si-O-Si groups are observed in the range of 807 cm −1 . The peak observed in the 462 cm −1 region is related to the bending vibrations of the Si-O-Si groups. 59 Aer the functionalization of KIT-6 with 3chloropropyltrimethoxysilane (CPTMS), new peaks appear. The existence of anchored CPTMS is conrmed via the C-H stretching vibrations at 2925 cm −1 . 63,64 Curve (c) shows bands at 1470 cm −1 due to C-C, and at 1648 cm −1 due to C]N, which proves that the s-methyl isothiouronium sulfate (SMTU) molecules have been bonded on the KIT-6@CPTMS surface. The signal of the C]N functional group shied from 1648 cm −1 to 1636 cm −1 in the FT-IR spectrum of KIT-6@SMTU@Ni. This change is assigned to the coordination of s-methyl isothiouronium with Ni nanoparticles. 36,61,64 This result indicates that the Ni nanoparticles were successfully immobilized on KIT-6.

N 2 adsorption-desorption isotherm studies
The nitrogen adsorption method is a very valuable method to determine the physical properties of materials. This technique is generally used to determine the area, volume, and diameter of pores, describing the size distribution of pores of mesostructured materials.
For the nitrogen adsorption-desorption isotherms, type IV isotherm distributions with an H1 hysteresis loop for the mesoporous KIT-6 material and KIT-6@SMTU@Ni catalyst are  Total proven volume (cm 3 g −1 ) KIT-6 581.14 0.7734 KIT-6@SMTU@Ni 509.65 0.6965  Fig. 7. 65,66 Also, BJH diagrams of KIT-6 and the KIT-6@SMTU@Ni catalyst are shown in Fig. 8. Both patterns prove the existence of mesoporous materials and show the uniformity of the synthesized mesoporous KIT-6 and catalyst. Moreover, the stability of the pattern in the functionalized KIT-6 shows that the KIT-6 structure is well preserved aer functionalization. Besides, the stabilization of the Ni-complex does not change in the structure of KIT-6. Nitrogen adsorption-desorption data, indicating the specic surface area (581.14 m 2 g −1 ), pore volume (0.7734 cm 3 g −1 ) and average pore diameter (5.32 nm), for the used KIT-6 support and the prepared catalyst, are given in Table 1. As shown in the table, the data for surface area and pore volume for KIT-6@SMTU@Ni decreased compared to KIT-6 due to loading of the SMTU@Ni complex in the KIT-6 pores.
In fact, modifying the surface of KIT-6 reduces the space of the pore, which changes the volume and surface area of the pore. However, the regular structure of the pores is preserved in the composition of the initial mesoporous KIT-6 aer surface correction.

Catalytic studies
Aer the catalyst synthesis and identication, in order to investigate the catalytic activity of KIT-6@SMTU@Ni as a recoverable catalyst, we employed it for the synthesis of tetrazoles (Scheme 2) and pyranopyrazoles (Scheme 3). Optimization of the reaction conditions for the synthesis of 5-substituted 1H-tetrazoles (considering the solvent effect, amount of catalyst, and temperature) was performed for the reaction of benzonitrile to the corresponding tetrazole as a model reaction. Before optimizing the temperature and amount of the catalyst used, it is necessary to select a suitable solvent; therefore, several solvents, such as dioxin, DMF, PEG, H 2 O, and DMSO, were used. The results showed that PEG-400 can be a suitable solvent for the reaction, which can provide the conditions for the reaction in a shorter time and higher efficiency. Subsequently, the efficacy of the amount of catalyst on the rate of progression was also investigated. Moreover, the effect of temperature on the reaction rate was also investigated. It was observed that the reaction progressed well with 20 mg of catalyst at 120°C. Therefore, 1.2 mmol of sodium azide, 1 mmol of benzonitrile, 20 mg of KIT-6@SMTU@Ni catalyst (0.46 mol%), and PEG solvent at 120°C were selected as the best reaction conditions ( Table 2, entry 2). To evaluate the efficiency of this synthetic method, various derivatives of tetrazoles were synthesized by reacting various nitriles with sodium azide. These results are summarized in Table 3. A reaction mechanism for the synthesis of 5-substituted 1Htetrazoles is shown in Scheme 4. As shown in Scheme 4, the nitrogen atom of the nitrile primarily coordinates to the metal (Ni) of the catalyst, to pull the p electron density onto the N atom and make it more nucleophilic. This interaction forms intermediate I. Indeed, KIT-6@SMTU@Ni acts as a Lewis acid, which activates the nitrile groups via coordination. Aerwards, it reacts with sodium azide to form intermediate II. The protonolysis produces tetrazole as the nal product, and the catalyst is released for the next run of the reaction. [67][68][69] It should be noted that there are different methods for preparing pyranopyrazoles; but most of these methods have limitations such as incompatibility with the environment, reaction time, high cost, production of by-products, purication problems, selectivity, and low productivity of products. Therefore, it will always be important to provide methods that can solve these problems.
To optimize the reaction conditions for the synthesis of pyranopyrazoles, diverse parameters, such as the amount of catalyst and different solvents in the four-component concentrations of malononitrile (1 mmol), 4-chlorobenzaldehyde (1 mmol), ethyl acetoacetate (1 mmol) and hydrazine hydrate (1 mmol) were investigated as the model reaction (Table 3). In order to select the appropriate solvent for the synthesis of pyranopyrazoles using the KIT-6@SMTU@Ni catalyst, the 4-chlorobenzaldehyde reaction was investigated as a sample reaction in the presence of a constant amount of catalyst at various temperatures using various solvents, such as water, ethanol and Scheme 4 The proposed mechanism for the synthesis of tetrazoles in the existence of KIT-6@SMTU@Ni.    (Table 5). A reaction mechanism for the formation of pyranopyrazoles is suggested in Scheme 5. Primarily, the KIT-6@SMTU@Ni catalyst activated the carbonyl groups of ethyl acetoacetate. Aerward, the carbonyl groups of ethyl acetoacetate were exposed to nucleophilic attack by (NH 2 groups of) hydrazine hydrate with two nucleophilic sites.
Aer nishing the reaction, the catalyst was separated, washed with hot ethyl acetate, dried at 60°C and then recycled for the subsequent reaction run. The catalyst can be recycled over 5 runs without considerable loss in its activity. The FT-IR spectrum of the recycled catalyst aer ve cycles does not show any considerable change, compared to the fresh catalyst, which is evidence for the chemical structure of the catalyst remaining stable during the reaction (Fig. 10).

Comparison of the catalyst
As shown in Table 6, to check the performance of KIT-6@SMTU@Ni as the catalyst for the production of the substituted 1H-tetrazoles and pyranopyrazoles, the obtained results were compared to the previously reported results of other catalytic systems in the literature. The attractive features of this newly proposed catalyst are short reaction times, recoverability, high reaction yield, recyclability by simple ltration, available and inexpensive starting materials and lack of toxicity. Fig. 9 Recyclability study of the KIT-6@SMTU@Ni catalyst in the model tetrazole and pyranopyrazole reactions. Fig. 10 FT-IR spectra of recovered KIT-6@SMTU@Ni. Ni-MP(AMP) 2 @Fe-biochar 4 97 36

Conclusions
In this study, the synthesis of an SMTU@Ni complex immobilized onto the surface of mesoporous KIT-6 as a new, reusable, and efficient catalyst has been presented. The structure of the catalyst was studied using XRD, EDX, TGA analysis, BET measurements and SEM, and FT-IR spectroscopy. The SEM images of the samples show that the particles are spherical with sizes of about 30 nm for KIT-6 and 40 nm for KIT-6@SMTU@Ni. The XRD pattern shows that the tridimensional symmetric cubic structure of the KIT-6 material remains unchanged aer Ni(II) modication. Moreover, the EDX spectra of the synthesized catalyst show the presence of silicon, oxygen, carbon, nitrogen and also nickel species. The FT-IR analysis proved the presence of Ni(II) species in the framework of mesoporous KIT-6. The BET studies show that the incorporation of nickel into the silica walls decreased the surface area and pore volume parameters. Moreover, its catalytic activity was investigated in two important syntheses of tetrazoles using nitrile, NaN 3 , and PEG solvent at 120°C, and pyranopyrazoles using aldehyde, hydrazine hydrate, malononitrile, ethyl acetoacetate at reux and temperature 80°C under ethanol : water solvent conditions. High yields of products, an eco-friendly protocol, short reaction time, simple operating procedure, the use of a novel recyclable catalyst and facile separation of the catalyst by simple ltration are additional benets from this protocol.

Conflicts of interest
There are no conicts to declare.